Handling of paper and other printing media in printers and copiers often involves having the paper travel with or around a roller. This generally occurs, for example, in printing an image on paper using an impression roller, especially for heated printing, as well as in some systems flipping paper over before printing a second side, or in flipping over a two-sided original page that is being copied, and in some systems conveying paper from an input tray to an output tray. Because paper tends to retain its curl to some extent after it is passed around a roller, printers and copiers use various methods of decurling paper, so that the final printed page, as well as the original paper being copied in a copier, is flat. U.S. Pat. No. 5,450,102, to Ishida et al, describes a decurling mechanism for a printer in which paper is decurled by bending it around a roller in the opposite direction from the direction in which it acquired its curl, but along the same axis.
Other printers decurl paper by bending it along an axis orthogonal to the original axis along which it was curled. For example, guides, mounted on walls to the sides of the paper, press against the paper from the sides, bending the paper as it falls into an output area. In some printers, the reaction force of the paper on the guides pushes the guides out of the way, thereby limiting the force that the guides exert on the paper. In some of these printers, there are counter-weights on the guides, so that the guides swing back up, to press against the next sheet of paper, after the paper falls into the output area. If the counter-weights nearly balance the weight of the guides, then the force required to push the guides out of the way is very small, and the guides exert only a very small force to bend the paper. Such an arrangement may be advantageous particularly when the paper is very light-weight and bends easily. However, the counter-weights take up room.